Field of the Invention
The present invention relates to coupling mechanisms for use with rotary cutting tools and, more particularly, to rotary cutting tools including such coupling mechanisms.
Background Information
Historically, end mills for metal cutting machinery were produced as a single unit, comprising a fluted cutting portion and a cylindrical or conical shank portion sized to fit a machine spindle. However, increasing global pricing of modern tool alloys along with recently developed intricate surface treatments have made such single units less economical, as the expensive shank material is generally wasted. It has therefore became common practice to produce a separate cutter made of high quality alloy or sintered carbide, which is then concentrically attached to the end of a reusable steel shank.
It is highly desirable that the cutter be easily replaced, upon wear, while leaving the shank in the machine spindle, such that no further adjustments are required after cutter replacement. A major requirement related to such accurate milling applications is that each replacement cutter be repeatedly, accurately, centered to the true spindle axis of rotation and axially positioned correctly.
One basic method currently in use for joining the cutter to the shank is disclosed for example in U.S. Pat. No. 5,114,286, which teaches an interchangeable cutting tool alignment and positioning system comprising a first tool segment having a male coupler and a second tool segment having a female coupler. The male coupler comprises a pilot in the form of first cylindrical mating surface, a concentric aligner in the form of second cylindrical mating surface spaced apart from the pilot, a male thread extending between the pilot and the concentric aligner and an axial stop in the form of planar surface. The female coupler comprises a pilot bore in the form of complementary cylindrical mating surface, corresponding to the cylindrical mating surface of the pilot, a concentric bore in the form of a complementary cylindrical mating surface corresponding to the cylindrical mating surface of the concentric aligner, a female thread extending between the pilot bore and the concentric bore, and an axial stop in the form of complementary planar surface.
The described pilot, concentric aligner, pilot bore and concentric bore, are necessary because the threaded coupler by its own is not sufficiently accurate for such repeated replacement of cutters.
Further improvements to the above basic concept are also known. For instance, U.S. Pat. No. 6,485,220 discloses a frustoconical radial alignment instead of a cylindrical alignment, as well as a strengthened thread root and U.S. Pat. No. 7,329,073 describes adjacent axial and radial stop surfaces.
Nevertheless all the above described solutions suffer from restrictive production requirements. Typical production tolerances of the cylindrical mating surfaces on the cutter and shank, sufficient for satisfying the need of replaceable cutters falling repeatedly in the desired range of concentricity and axis position, are less than 5 micrometers. Such close tolerances necessitate an additional grinding process.
Furthermore, sintered carbide cutters by their nature are very hard yet also very brittle. Direct coupling of the hard cutter to the steel shank imposes stresses on the coupling where the two different materials engage. More particularly, in cases where a carbide cutter is threaded into a steel shank, failure of the connection is likely to occur at or near the base of the threaded portion of the carbide cutter, which commonly also damages the steel shank, rendering it unsuitable for reuse.
Hence there is room for improvement in coupling mechanisms for use with rotary cutting tools and also to rotary cutting tools including such coupling mechanisms.